1. Field of Invention
This invention relates generally to atmospheric optical and solar energy measuring systems and more specifically to a method and apparatus for measuring optical conditions of the atmosphere in real time, estimating spectral solar radiation, and comparing with standard conditions.
2. Description of the Prior Art
The sun, for practical purposes, provides an essentially constant source of solar energy. However, it is known that a variety of factors or parameters affect the amount and character of solar energy that reach any particular place on the surface of the earth. Thus, effective useable solar energy, such as for powering photovoltaic cells and similar uses, varies as a function of a variety of parameters, including relative position of the sun to the place on earth where the photovoltaic cell or other use is positioned, the atmospheric conditions, and the like.
More specifically, it has been shown that the performance characteristics of photovoltaic devices (e.g., short-circuit current, efficiency factor, open circuit voltage, and power output) vary as functions of atmospheric optical parameters and the spectral solar irradiance incident on the photovoltaic devices. Consequently, when photoelectric devices are tested in outdoor ambient conditions, the performance data will contain a certain amount of variability from one place to the next and from one time to the next.
For example, it is known that incident spectral solar irradiance conditions are quite variable because of such parameters as extraterrestrial solar irradiance characteristics, the optical transmittance properties of the intervening atmosphere, the slant path of the direct-beam sunlight through the atmosphere, and the reflectance properties of the ground as viewed by the photovoltaic device. In other words, the inherent temporal and spatial (geographical) variability of atmospheric optical conditions, along with variations in relative air mass, which is a function of location, time of day, and time of year, creates variables in the spectral solar irradiance incident on photovoltaic devices tested outdoors.
To compare quality and performance of various photovoltaic devices, it is necessary to have some standard reference conditions against which, or in which, all photovoltaic devices can be tested or to which individual test results can be correlated. One method that persons who work in the photovoltaic field have used to correct outdoor performance measurements to reference conditions or translate such results to typical conditions or both is known as the reference cell method. In this reference cell method, the short-circuit current density is assumed to vary linearly with total solar irradiance, which is really not entirely accurate. Also, the translation of the current to a reference solar irradiance condition assumes that the fill factor of the photovoltaic device is independent of irradiance, which also is not entirely correct. Further, the spectral response of the device being tested, including submodules and modules, must be identical to the spectral response of the reference cell, which is unrealistic. Finally, in the reference cell method the photovoltaic device performance is measured with respect to a reference solar irradiance spectrum, temperature, and irradiance level. However, these parameters vary in actual operating conditions. These shortcomings in the reference cell method are especially pertinent to newer photovoltaic devices developed more recently, which employ a variety of materials and contain multiple band gaps.
Another approach to standardizing tests results is to simply measure the spectral solar irradiance incident on the photovoltaic device simultaneously with the performance testing. Such measurements can be made with a suitable spectroradiometer. Then, knowing the spectral distribution of the solar irradiance and the spectral response of the device being tested, the approach is to translate device performance measurements to standard or typical conditions. This approach, of course, assumes that one knows the spectral solar irradiance for standard or typical conditions.
However, spectroradiometers are expensive devices that are difficult to operate and even more difficult to calibrate. Therefore, many photovoltaic researchers find it difficult to operate spectroradiometers, and, even if they learn to operate them, they often cannot afford one. It is also very difficult to use data obtained from spectral radiometers to calculate photovoltaic device performances.
Therefore, recognizing these issues and problems, especially for advanced photovoltaic devices, there has become an identified and articulated need for an improved and less expensive standardized system for measuring and comparing atmospheric optical and spectral solar-irradiance conditions. Such a system should provide economical and simple measurements and readily available, easy-to-use instrumentation to characterize atmospheric optical properties and spectral solar-irradiance conditions during outdoor performance testing of photovoltaic devices. Prior to this invention, there was no system available to fill this need for the community of people who work in the field of research and development of photovoltaic devices.